1. Field of the Invention
The invention relates to a coating thickness gauge for measuring the thickness of a coating of chromic oxide on a chromium layer on a substrate using polarized light, and to a method of measuring the thickness of such a layer using the gauge.
2. Description of the Prior Art
A coating thickness gauge is known from EP-A-249235 which is based on the principles of ellipsometry. The gauge comprises a light source for generating linear polarized light, fixed splitting means for splitting elliptically polarized light reflected through the coating into a number of beams, a measuring apparatus for measuring the intensity of each of the beams and calculation means for calculating the thickness of the coating from the intensities measured.
In use of this gauge an incident beam of linear polarized light is directed onto a coating to be measured. The part of the incident beam with a direction of polarization parallel to the plane of incidence, that is the plane through the incident beam and the reflected, beam, is reflected differently both in phase and in amplitude from the part of the beam with a direction of polarization perpendicular to the plane of incidence. The result is a reflected beam which, in general, is elliptically polarized; that means that the electrical field strength in a plane perpendicular to the direction of the reflected beam describes an ellipse. The ratio r.sub.p /r.sub.s between the complex amplitude reflection coefficients in the parallel direction and in the perpendicular direction is a measure for the thickness of the coating. This ratio is measured by making use of the relation r.sub.p /r.sub.s =tan .PSI. exp j .DELTA. and measuring .PSI. and .DELTA. and one intensity. In this relation .PSI. is the azimuth angle and the angle .DELTA. is calculated from the ratio between the long axis and the short axis of the ellipse, known as the ellipticity.
In the gauge of EP-A-249235 the measurement is made by dividing the reflected beam into at least three partial beams using semi-transparent mirrors as means of splitting. The partial beams are each monitored by an analyzer, each analyzer having a different angle of polarization. The intensity of the partial beam in each of the three directions of polarization is measured using a measuring apparatus suited to the purpose, such as a photodetector.
From the ratio of the three intensities in different directions of polarization and using the means of calculation, the azimuth angle .PSI. and the ellipticity of the ellipse may be calculated. From these, making use of the above relation the thickness of the reflecting coating may be obtained.
An inconvenience of this known apparatus is that the optical system needed is relatively complicated and puts great demands on optical alignment. This is a particular disadvantage in a production environment e.g. for coated steel sheet. Another inconvenience is that for a precise thickness measurement the optical properties of the components used must be known exactly. This inconvenience applies particularly to semi-transparent mirrors because semi-transparent mirrors affect the direction of polarization of both the transmitted and the reflected beams. This causes an error in determining the ellipse. Another inconvenience is that a great number of calculations have to be made for obtaining the thickness of the reflecting coating.
A thickness gauge provided with a polarizing beam splitter is known from EP-A-278577. This describes a coating thickness gauge for measuring the thickness of a coating of protein on a substrate. In the coating thickness gauge described the incident beam is thrown onto the coating of protein at the Brewster angle and by means of a polarizing beam splitter the reflected beam is split into a parallel partial beam R.sub.p and a perpendicular partial beam R.sub.s.
Therefore, this coating thickness gauge only gives information about the azimuth angle .PSI. and is not suitable for measuring the thickness of a coating of chromic oxide.
Making use of the formula M=(R.sub.s -mR.sub.p)/(Rs+mR.sub.p) in accordance with the publication, M can be calculated an M is a measure for the coating thickness. The coating thickness gauge described is intended particularly for use when the incident and the reflected beams vary in intensity, for example as a result of an absorbing substrate through which the light must pass in order to reach the coating to be measured. The Brewster angle as angle of incidence in the coating thickness gauge described is only significant for substrates which have a true refractive index. In the case of a chromic oxide coating on chromium, a Brewster angle is not defined because chromium has a complex refractive index.